As is known, so-called "ball" valves are substantially comprised of a valve body, with a cylindrical cavity to which two or more ducts are connected for the fluid to be distributed or intercepted, and a ball provided with a longitudinal axial duct and capable of being operated from the outside, located within the cylindrical cavity and capable of being rotated so as to set the axial duct in alignment with the direction of the fluid for the passage thereof, or conversely with the duct set transverse relative to the direction of the fluid for the interception thereof.
The principal problems associated with the use of such valves are substantially centered on the sealing rings which surround the ball and are required to maintain a seal between the inlet and the ports in the two directions and between both ports and the axis of control toward the exterior.
Such sealing rings are subjected to pressure between the fixed part and the moving part, for which reason any wear likely to give rise to clearances which cannot be remedied should be avoided.
In particular, the sealing rings located upstream and downstream of the ball and in the direction of flow of the fluid are subjected to the erosive action of the fluid, particularly during closure and opening of the ball; during such operations, the axial duct of the ball which is rotated limits the inflow of fluid via the ball, but does not obstruct it completely; therefore, since the outflow channel is still open, there is encountered a marked increase in the velocity of the fluid with generation of turbulence acting on the upstream and downstream sealing rings, ultimately causing erosion thereof with resulting loss of sealing.
This condition is further aggravated by the fact that the fluids to be intercepted are normally supplied at high pressure and at high temperatures, as in the case of saturated steam, and the wear is more apparent in regard to the downstream gasket which is more likely to be exposed to such erosion and which is responsible for maintaining a seal when the valve is open.